The reaction between nitric oxide and lipid peroxyl radicals has been proposed to account for the potent inhibitory properties of nitric oxide toward lipid peroxidation processes; however, the mechanisms of this reaction, including kinetic parameters and nature of termination products, have not been defined. Here, the reaction between linoleate peroxyl radicals and nitric oxide was examined using 2,2'-azobis(2-amidinopropane) hydrochloride-dependent oxidation of linoleate. Addition of nitric oxide (0.5-20 micromole) to peroxidizing lipid led to cessation of oxygen uptake, which resumed at original rates when all nitric oxide had been consumed. At high nitric oxide concentrations (> 3 micromole), the time of inhibition of chain propagation became increasingly dependent on oxygen concentration, due to the competing reaction of oxygen with nitric oxide. Kinetic analysis revealed that a simple radical-radical termination reaction does not account for the inhibition of lipid oxidation by nitric oxide, and at least two molecules of nitric oxide are consumed per termination reaction. A mechanism is proposed whereby nitric oxide first reacts with lipid peroxyl radical to form LOONO. Following decomposition of LOONO to lipid peroxyl radical and NO2, a second nitric oxide is consumed via reaction with LO, with the composite rate constant for this reaction being k=7 x 10(4) M(-1) s(-1). At equal concentrations, greater inhibition of oxidation was observed with nitric oxide than with alpha-tocopherol. Since nitric oxide reacts with lipid peroxyl radical at an almost diffusion-limited rate, steady-state concentration of 30 nM nitric oxide would effectively compete with endogenous alpha-tocopherol concentrations (about 20 micromole) as a scavenger of lipid peroxyl radical in the lipid phase. This indicates that biological nitric oxide concentrations (up to 2 micromole) will significantly influence peroxidation reactions in vivo.